Much of the research and development effort in the field of automotive finishes is currently directed to the search for coating compositions and methods of applying such compositions which not only will eliminate, or nearly eliminate, the release of organic solvents during heat curing but also will produce at a commercially feasible cost, coatings at least comparable in appearance and durability to conventional coatings.
A substantial amount of current research effort is likewise directed to the development of clear coat/color coat automotive finishes. It has been found that an excellent appearance, with depth of color and with metallic glamour, can be obtained by applying a transparent coat over a pigmented coat. Unfortunately, the durability of these transparent clear coats has left much to be desired. Often, checking, cracking, and flaking occur after relatively short periods of exposure to weathering, necessitating costly refinishing.
One solution to the solvent emission problem has been the replacement of liquid coating materials with coating materials in the form of dry, particulate solids, commonly called "powder" coatings. These compositions contain very low concentrations of volatile solvents, i.e., of the order of 2 percent or slightly higher, substantially less than any other paint system.
From an environmental standpoint, powder coatings have much to recommend them. Inherent in their use, however, are certain problems of production and application which have retarded the extent of their adoption. One problem occurs when powder coatings are used in conjunction with particulate metal particles, e.g., aluminum flakes. Automobiles coated with a so-called "metallic" finish, i.e., a topcoat of enamel or lacquer in which aluminum flakes as well as conventional pigments have been dispersed, have found wide acceptance in the marketplace. For the most part, the problems incidental to employing aluminum flakes in conventional liquid paints have been solved through years of experimentation and use. The problems associated with the use of aluminum flakes in dry powder are far more complex, particularly where some type of pulverizing step is involved in the paint manufacturing process or where electrostatic spray techniques are used to apply the paint to a substrate. Also, although increased use of powder coatings and improved manufacturing methods will undoubtedly result in a reduction of the present cost of quality powder coatings, the cost of producing such coatings in all of the colors demanded in the marketplace may continue to be prohibitive.
In view of the problems associated with colored powder coatings, particularly those containing metallic flakes, one approach has been to utilize a clear coat/color coat system wherein the transparent clear coat is composed of a nonpigmented powder coating while the color coat, often metallic, is composed of a conventional liquid paint. Such a system possesses a number of advantages. Use of a powder coating for the clear coat reduces the solvent emission level considerably, and, if desired, a water-based or high-solids coating material can be used for the color coat in order to further reduce the total emission level. Thus, an environmentally-acceptable finish can be achieved without sacrificing appearance or metallic glamour. Too, the production of nonpigmented powder coatings is markedly less complex and less expensive than the production of pigmented powder coatings.
Clear coat/color coat systems consisting of a powder clear coat over a conventional liquid color cost are known in the art, as shown by Camelon et al., U.S. Pat. No. 3,953,644, issued Apr. 27, 1976. However, such systems are not reinforced against the destructive effects of outdoor weathering and are thus susceptible to the checking, cracking, and flaking which commonly beset clear coats.
Conventional ultraviolet light screeners have sometimes been added to liquid clear coats in an attempt to retard the degradation caused by weathering, as shown by La Berge, U.S. Pat. No. 3,407,156, issued Oct. 22, 1968. Many of these conventional screeners are unsuitable for use in a powder coating because they lack special requirements, e.g., the capability of being ground to small particle size, of melting readily at the standard bake temperature for powder finishes, or of having sufficient permanence to remain in the coating film during baking and subsequent outdoor exposure. In addition, the use of conventional ultraviolet light screeners is in some polymer systems less than satisfactory: the durability of certain clear coats so reinforced will be increased for a short period of time, but not to the extent required for a practical automotive finish.
Among the acrylic polymers which provide high quality automotive finishes are those containing glycidyl methacrylate. The use of glycidyl methacrylate in powder coating compositions is known in the art, as shown by Victorius, U.S. Pat. No. 4,027,066, issued May 31, 1977. These compositions, like the aforementioned powder clear coats of the art, are not reinforced against weathering. Furthermore, these compositions are ordinarily pigmented and contain cellulose acetate butyrate, useful as a dispersant for the organic pigments commonly encountered in powder coatings. While cellulose acetate butyrate imparts goniochromatism, i.e., metallic two-tone, to a metallic colored automotive finish, it is less suitable for nonpigmented finishes. The presence of cellulose acetate butyrate in an acrylic system creates a faint cloudiness or haze which is of no consequence in pigmented coats but can be detected in nonpigmented coats and detracts from the overall appearance of the finish. The presence of cellulose acetate butyrate also tends to have a somewhat detrimental effect upon the cold/crack resistance and the outdoor durability of a coating.
Thus, there exists a felt need for a nonpigmented acrylic powder coating composition which will provide a clear coat characterized by both excellent appearance and the capability to adequately withstand long periods of outdoor weathering. In particular, there is need for an acrylic powder coating possessing the attributes of the known glycidyl methacrylate powder coatings but overcoming their deficiencies in order to provide a clear coat finish with excellent cold/crack resistance and superior clarity and durability.